This invention relates to the hot water process for extracting bitumen from bituminous sands. More particularly it relates to a method of controlling the rate of tailings withdrawal through the outlet of the primary separation vessel used in the process.
A large proportion of the world's known hydrocarbon reserves exists in the form of bituminous sands. One large deposit of this material is found along the banks of the Athabasca River in Alberta. It exists in the form of waterwet grains of sand, sheathed in a film of bitumen. In treating the sands to recover commercially useful products, it is first necessary to separate the bitumen from the water and solids.
The method presently employed to extract the bitumen from the mined sands is known as the hot water process. In the first step of this process, bituminous sands, hot water, a minor amount of a dispersant, such as NaOH, and steam are fed into a rotating tumbler and mixed therein. The hot water is supplied at a temperature of about 180.degree. F and in amounts sufficient to supply a slurry containing about 20 - 25% by weight water. The dispersant is typically provided in an amount of 0.025% by weight of tar sand. The residence time within the tumbler is nominally four minutes and the exit temperature of the slurry is about 180.degree. F. While in the tumbler, the tar sand disintegrates and the bitumen particles are liberated from the sand.
The tumbler product is passed through a screen to remove lumps and rocks and is then flooded with additional hot water to further disperse the sand and bitumen particles. A typical flooded slurry will have a composition of 7% bitumen, 43% water and 50% solids, and its temperature will be about 160.degree. F - 180.degree. F.
The flooded slurry is then continuously fed into a primary separation vessel. This vessel is conventionally a cylindrical settler having a conical bottom. In the vessel, most of the large sand particles (i.e. plus 200 mesh) fall to the bottom and leave through an outlet as a primary tailings stream. Most of the bitumen particles rise to the top of the vessel and form primary bitumen froth. This froth overflows the vessel wall into a launder for removal. A middlings stream, typically comprising about 77% water, 21% solids and 2% bitumen, is continuously withdrawn from the intermediate zone of the primary vessel. The middlings stream is processed in a sub-aerated secondary recovery flotation cell to produce secondary froth and a secondary tailings stream.
For purposes of this specification, "fine solids" is understood to mean -325 mesh particulate matter.
The fine solids content of bituminous sands varies widely. For example, in a regular or "low fines" bituminous sand, less than about 15% by weight of the total solids are fine solids while in a "high fines" bituminous sand, greater than about 20% of the total solids are fine solids.
Heretofore it has been known that high fines sands are difficult to treat in the hot water process and yield relatively poor bitumen recoveries.
At this point it is useful to digress and review how the primary separation vessel is operated in accordance with the prior art. The bituminous sands slurry is usually fed to the vessel at a generally constant rate, although, of course, its composition varies since the sands themselves vary in composition. Three product streams are produced from the vessel. The first of these is the froth product, which overflows the vessel rim and drops into a circumferential launder. The second is the middlings stream which is withdrawn by a variable-speed pump and is pumped to the secondary recovery cell. The level of the froth-middlings interface is monitored by a sensing device and the rate of middlings withdrawal is controlled in response to this measurement with the aim of keeping the position of the interface constant. The third product is the tailings stream. Its rate of withdrawal is controlled by throttling means, such as a valve or variable-speed pump, in the outlet line. The operation of the throttling means is regulated by a torque-sensing device which measures the torque generated in the shaft of the vessel's sand rake. The torque measurement is assumed to be related to the position of the surface of the sand bed within the vessel. More particularly, as the sand bed builds up, it begins to cover the rake, thereby increasing the torque developed in the rake shaft. Now, the throttling means are operated to maintain a sand seal at the outlet and to maintain the tailings as dry as possible (i.e. in the order of 70% solids) to minimize oil losses with the tailings.
As the composition of the tar sand slurry entering the vessel varies, it is necessary to manipulate the throttling means on the middlings and tailings lines to keep the froth-middlings and middlings-sand interfaces positioned at pre-determined desirable levels.
When working with low fines feed, the torque-sensing system works satisfactorily. The sand bed seems to be well-defined and its dragging effect on the sand rake varies directly with the extent to which the prongs of the rake are buried in it.
As previously pointed out, however, when the vessel is fed high fines slurry feed, difficulties arise. It appears that a sand bed having a firm upper layer is not developed. Thus the position of the bed surface is not accurately indicated by the torque-sensing device mounted on the rake shaft.
In practice, one finds that the rake torque measurement remains generally low for a period of time as high fines slurry feed is processed in the vessel. As a result, the throttling means on the tailings outlet is kept in a constrictive condition. Suddenly, however, the rake torque increases dramatically, indicating that the rake has become buried to a substantial extent. When this occurs, the tailings outlet throttling means is adjusted and tailings are withdrawn at a rapid rate. This causes the froth-middlings interface to drop, thereby triggering constrictive adjustment of the middlings line throttling means. After a quantity of tailings has been removed, the rake torque drops off quickly and the tailings throttling means sharply reduces tailings withdrawal. In order to maintain the froth-middlings interface at the desired level, middlings withdrawal is then accelerated. "Short circuiting" of the vessel operation may occur, as bitumen is drawn out through the middlings line.
From the foregoing, it will be understood that provision of high fines slurry feed to a primary separation vessel, controlled in accordance with the prior art, leads to:
1. Unstable operation of the primary separation vessel and surging of froth and middlings product streams, which is undesirable as it affects the operations of downstream units;
2. Short-circuiting of the primary separation cell, with the result that a high proportion of the bitumen may be produced as secondary froth - this is undesirable as this froth is more heavily contaminated with solids than primary froth, due to the vigorous aeration which is practised in the secondary cell; and
3. Settling of solids in the middlings and tailings outlet lines during the periods when the throttling means controlling flow through those lines are constricted, which can lead to plugging of the lines.